1. Field of the Invention
The present invention relates to a disc used as a recording medium on which there are recorded various kinds of information signals and, more particularly, to a disc of a type such that the disc is chucked to a spindle in a magnet-chucking fashion.
2. Description of the Prior Art
Optical discs such as a magneto-optical disc or the like have long been known as a recording medium on which there are recorded various kinds of information signals.
As an optical disc that is chucked to a disc rotating spindle of an optical disc recording and reproducing apparatus in a so-called magnet-chucking fashion, the assignee of the present application has previously proposed Japanese Utility Model Application No. 62-199240 (Japanese Laid-Open Utility Model Publication No. 1-107072), etc.
An outline of this conventional optical disc will be described with reference to FIGS. 1 to 3.
As illustrated, an optical disc 1 generally depicted by reference numeral 1 is comprised of a disc body 2 and a disc hub 5 having a resin ring 4 unitarily molded with a circumferential edge portion of a metal plate 3. The disc body 2 has a disc-shaped configuration and is made by molding a synthetic resin, e.g., polycarbonate resin. One or both surfaces of the disc body 2 are used as recording surfaces in which recording tracks are formed in the circumferential direction thereof. A circular central aperture 2a is bored through a central portion of the disc body 2 as shown in FIG. 2.
The metal plate 3 of the disc hub 5 is formed as a disc from a magnetic material such as an iron plate or the like. A centering aperture 6 whose diameter is smaller than that of the central aperture 2a of the disc body 2 is coaxially bored through the central portion of the metal plate 3 of the disc hub 5. Then, the resin ring 4 is unitarily molded with the circumferential edge portion of the metal plate 3 by a so-called outsert molding method, thereby constructing the disc hub 5.
An annular welding rib 7, which is used to weld the disc hub 5 to the disc body 2, is formed on the resin ring 4 on its surface opposing the disc body 2. Further, a plurality of guide ribs 8 having a leg-piece configuration, are fitted to the inner circumferential surface of the central aperture 2a of the disc body 2, and project from the inner circumferential portion of the resin ring 4.
When the disc hub 5 is attached to the disc body 2, the resin ring 4 is located on the surface of the disc body 2 in such a manner that the welding rib 7 thereof opposes the surface of the disc body 2 while the guide ribs 8 are fitted into the inner circumferential surface of the central aperture 2a. If an ultrasonic horn (not shown) is pressed on the resin ring 4 and an ultrasonic vibration is applied to the resin ring 4 during a predetermined period of time under that state, then the welding rib 7 becomes molten to weld the disc hub 5 to the disc body 2.
In this fashion, the disc hub 5 is attached to the disc body 2 in a so-called centering fashion such that the centering aperture 6 of the metal plate 3 accurately coincides with the center of the recording tracks on the disc body 2. While the disc hub 5 is welded onto the upper surface of the disc body 2 as shown in FIG. 2, the disc hubs 5 are welded onto both upper and lower surfaces of the disc body 2 in actual practice.
The optical disc 1 thus arranged is generally preserved under conditions wherein it is accommodated within a disc cartridge. When the optical disc 1 is loaded onto an optical disc recording and reproducing apparatus, the centering aperture 6 of the disc hub 5 is inserted into a spindle S made of a metallic material such as a stainless steel or the like and is thereby positioned. Also, the metal plate 3 of the disc hub 5 is chucked in a magnetic-chucking fashion to a disc table T which is provided at the outer periphery of the spindle S and around which there is embedded an annular magnet (not shown). Then, when the optical disc 1 is rotated by the spindle S, various information may be recorded on and reproduced from the optical disc 1 by means of an optical pickup (not shown).
However, because the spindle S is made from a metallic material, the conventional optical disc 1, in which the centering aperture 6 of the disc hub 5 is directly bored through the metal place 3, tends to produce very small metal powders when the metal plate 3 and the metal spindle S wear against each other during the magnetic-chucking process.
Also, because the spindle S must locate the optical disc 1 with high accuracy, a difference between the outer diameter of the spindle S and the inner diameter of the centering aperture 6 is generally set to a value as small as about 12 .mu.m.
Accordingly, if the inside of the centering aperture 6 and the surface on which the metal plate 3 is chucked to the disc table T are smudged by metal powders produced due to wear, then the normal chucking operation of the optical disc 1 to the spindle S may be hindered. If the optical disc 1 is rotated under the condition wherein the optical disc 1 is imperfectly chucked to the spindle S, then the optical disc 1, the disc hub 5, the spindle S, the optical pickup or the like will be damaged.
To solve the above problem, the assignee of the present invention has previously proposed a disc wherein a unitarily-molded inner peripheral resin ring is formed on the inner periphery of a metal plate of a disc hub by a wear-proof resin and a centering aperture is formed at the center of the inner peripheral resin ring, thereby preventing metal powders from being produced when the metal plate is subject to wear.
However, after the above disc has been in use for a long period of time, the centering aperture which is bored through the inner peripheral resin ring (made of the wear-proof resin) becomes worn by a very small amount, which unavoidably produces wearing powders. In addition, since the inner peripheral resin ring is made of resin, the inner peripheral resin ring tends to become charged with static electricity. Recent studies have revealed that, due to the static electricity charged, the inside of the centering aperture or the like tends to adsorb the above wearing powders, dust or the like, which hinders the normal chucking operation of the disc onto the spindle.